Capillary rheometer plunger pressure transducer and measurement technique

ABSTRACT

A capillary rheometer apparatus includes a housing, a plunger, the housing having a reservoir for receiving the plunger in a polymer melt, and a mechanism for blocking flow of the melt out of the reservoir. A driving mechanism is included for driving the plunger longitudinally within the reservoir to move one end of the plunger in contact with the melt. A diaphragm, which is coupled to the one end of the plunger, deflects in response to melt pressure in the reservoir. A mechanism, responsive to diaphragm deflection, determines pressure of the melt. Another mechanism is included for determining the temperature of the melt and a further mechanism is included for indicating longitudinal movement of the plunger. From such determinations, the PVT characteristics and the apparent shear viscosity characteristics of the polymer melt can be obtained.

RELATED APPLICATION

This application is a continuation-in-part application under 35 U.S.C.§120 of application Ser. No. 07/680,561 filed Apr. 4, 1991, now U.S.Pat. No. 5,209,107, entitled "CAPILLARY RHEOMETER PLUNGER PRESSURETRANSDUCER AND MEASUREMENT TECHNIQUE".

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a capillary rheometer for establishingcompressibility material properties and pertains, more particularly to acapillary rheometer which utilizes a pressure measurement plunger forsuch purposes.

2. Background

Various types of capillary rheometers are utilized in the polymerindustry to establish shear and temperature related material propertiesas well as compressibility properties. The theory of operation anddesign specifications for capillary rheometers are documented in U.S.Pat. No. 3,203,225.

Capillary rheometers generally operate by using a piston or plunger toforce melted polymers, that have been heated in a barrel passage,through a capillary die. The force based plunger-barrel capillaryrheometer utilizes a force sensor to measure the load or force appliedto the plunger and a displacement sensor to measure the plunger velocity(displacement/unit time) through the stationary barrel. The apparentshear viscosity of the melted polymer can be determined using knownrelationships for flow of polymer melts through the cylindrical or othercommonly used geometries. For example, wide thin slits or annulusgeometries may be used. The apparent shear viscosity of a polymer meltat a given melt temperature is determined using the ratio of wall shearstress divided by apparent wall shear rate, for the capillary of adefined geometry. The wall shear stress depends upon the plunger forcemeasured by the force sensor.

In addition to establishing shear and temperature related materialproperties, capillary rheometers can be modified to generate informationon the compressibility of polymer melts. In such an application, thepressure-volume-temperature (PVT) relationships, so called "equation ofstate" relationships, of a polymer melt can be determined using thecapillary rheometer in the following manner. The rheometer barrel isheated to a desired temperature in which polymer granules, pellets orpowder are loaded into the barrel and allowed to soften due to the heat.A plunger is used to apply various levels of pressure to the polymer viaweights, air pressure, mechanical pressure, or hydraulic pressure. Aknown diameter plunger with a force measuring sensor is used todetermine the pressure within the polymer melt. The temperature of thepolymer melt, and the volume of the polymer melt are determined as afunction of applied pressure. The specific volume of the polymer, atvarious pressures, is plotted against polymer temperature to describethe PVT behavior of the polymer.

There are, however, a number of errors associated with the meltedpolymer apparent viscosity data and compressibility data determinedusing the above mentioned method. With respect to viscositymeasurements, the shear stress and the apparent shear rate values haveerrors associated therewith. These errors will be described, inparticular, with reference to a prior art embodiment of the presentinvention, as illustrated in FIGS. 1 and 2.

Shear stress values will be in error if determined by means of a forcesensor, because the force at the top of the plunger is influenced by thefollowing factors which are not considered when the force sensor methodis employed:

1. The Pressure Drop in the Barrel: The barrel 6 of the capillaryrheometer is itself a capillary of given diameter and continuouslydecreasing effective length as the plunger 5 moves downward. The forcerequired to maintain flow through the barrel 6 (i.e., pressure dropalong barrel 6) can be significant, especially since the shear rateassociated with barrel flow is low, and melted polymers have relativelyhigh viscosities at low shear rates as most polymers are pseudoplasticin nature. The pressure drop is not considered by the force sensormeasurement and thus a resulting error occurs in capillary wall shearstress since the stress value calculated assumes all of the pressuredrop is due to the capillary itself. In addition, this error is not a"constant" at a given temperature and plunger 5 speed since theeffective length of the barrel 6 changes continuously.

2. Friction Between Plunger and Reservoir Wall: In order to minimize theflow of material back across the land of the plunger 5, the plunger 5must be fitted tightly within the barrel 6. The plunger 5 may berelieved some distance back from the melted polymer 9 interface,although enough tightly fitted land must remain to (i) limit the backflow of melted polymer 9 and (ii) align the tip of the plunger 5 in thebarrel 6. Low coefficient of friction plunger seals 8 are often used toreduce the back flow of the melted polymer 9.

The melted polymer 9 may stick to the wall of the barrel and may besheared between the wall and the plunger 5 as the plunger 5 moves. Theplunger 5 itself will rub against the barrel 6 wall unless it isperfectly straight, properly aligned, and has the correct dimensions.High pressures in the barrel 6, such as those encountered when workingwith viscous materials at high flow rates, could cause buckling of theplunger 5 within the barrel 6, and binding between the plunger 5 andbarrel 6. The dimensions of both the plunger 5 tip and barrel 6 willalso change when the operating temperature is changed. Changes inoperating temperatures could result in scoring of the barrel 6, or theopening (or closing) of the gap through which back flow can occur due tothermal expansion differences between the plunger and the barrel.Therefore, plunger friction errors are likely to occur.

Plunger 5 friction errors are typically estimated by removing thecapillary 12 and measuring the force required to force melted polymer 9from the barrel 6, and extrapolating this to force data to a zero barrellength. The method has been criticized since the friction errors varywith driving pressure and flow rate, and it is also time consuming.

3. End Errors: The entrance area of capillary 12 and barrel 6 exit areais a region where large stresses are developed due to the funneling ofthe melted polymer 9 as it emerges from the barrel reservoir, as well asregion where these stresses relax to their limiting value which occurssome distance along the length of the capillary 12 tube.

The exit pressure for capillary 12 has also been shown to be somewhatgreater than zero for viscoelastic polymers. The exit pressure is theresult of recoverable elastic energy within the melted polymer 9, causedby flow induced orientation of the polymer molecules during deformationupstream of the capillary 12 exit. Purely viscous materials have exitpressures of zero.

The end errors can be minimized using dies having longer L/D ratios,since they are essentially constants at a given temperature and rate,being independent of capillary 12 length. It should be appreciated thatthe end errors are a constant and, therefore, become smaller on apercentage basis as the capillary length increases. The errors can beeliminated using the procedure of classical hydrodynamics of plottingthe pressure drop measured over a system containing both an entranceregion and straight capillary 12 versus the L/R of the tube, for tubesof various lengths and constant diameter at each flow (or shear) rate.Extrapolation to a pressure drop at zero length gives the end effect interms of absolute pressure. Extrapolation to zero pressure gives the endeffect in terms of tube radii. An alternative method is to use a flowgeometry, such as a wide thin slit, for which the pressure drop withinthe rheometric region of the flow can be measured directly.

4. Temperature and Compressibility: It is generally assumed that thetemperature of the melted polymer 9 is constant, and that the meltedpolymer 9 is incompressible. Melted polymers 9 are in fact, however,compressible, and are generally viscous materials, having relatively lowthermal diffusitivities, indicating that the temperature of the polymeris likely to increase as it progresses through the measurement systemdue to viscous dissipation, to a degree depending on conductive heatloss. In order to minimize viscous heating and compressibility effects,short L/D capillaries 12 are recommended, provided end errors and barrel6 related errors can be accounted for, since their relative effect ismore significant for shorter capillaries 12.

5. Elastic Distortion: Elastic distortion of the barrel and polymerviscosity both change with temperature and pressure, plunger velocity,alignment and force. These changes as well as seal quality affect thecalculation of effective area used to determine the pressure generatedwithin the barrel of the capillary rheometer. The exact magnitude ofthese errors in a capillary rheometer are unknown although elasticdistortion and effective area calculations are well documented for deadweight piston gages.

The force/sensor pressure calculation does not take into considerationthe clearance area between the plunger 5 and the inner barrel wall. Theelastic distortion of the barrel and polymer viscosity change withtemperature and pressure and plunger velocity. These unaccounted forchanges cause errors in effective area and other related calculations.

6. Polymer Backflow/Leakage/Shear Rate Errors: The rate at which meltedpolymer 9 flows through the capillary 12 is assumed to be equivalent tothe value determined using the distance swept by the plunger 5 per unittime, assuming incompressibility and mass conservation. There willhowever be some leakage of material across the land of the plunger 5,since the pressure on the melted polymer 9 is greater than atmospheric.The amount of back flow will be determined by the quality of the plungerseal 8. Close, tight tolerances between the barrel 6 and plunger 5 willreduce leakage. An increase in the land length (contact area) will alsoreduce leakage. However, an increase in the number of plunger seals 8,or in the contact area between the plunger 5 and barrel 6, is alsoexpected to increase the magnitude of the plunger 5 barrel 6 frictionforce errors.

Force sensor pressure calculations do not take into consideration someleakage of the melted polymer across the plunger. There is, however,some leakage of the melted polymer across the plunger. Thus, errors areassociated with this calculation. By increasing the number of plungerseals or the contact area between the plunger and inner barrel wall,while it reduces the leakage, it increases the friction errors.

Accordingly, it is an object of the present invention to provide animproved capillary rheometer which eliminates the need for a force basedmeasurement plunger.

It is another object of the present invention to provide a capillaryrheometer in which accurate shear stress and apparent shear rate valuesfor a melted polymer can be determined.

It is another object of the present invention to provide a capillaryrheometer in which accurate compressibility data for a melted polymercan be determined.

It is another object of the present invention to provide a capillaryrheometer which will eliminate the need for corrective methods toaccount for errors due to the barrel pressure drop, friction between theplunger and inner barrel wall, end errors, temperature andcompressibility errors, elastic distortion errors, leakage errors andother related errors.

It is another object of the present invention to provide a capillaryrheometer which utilizes a pressure measurement plunger.

It is another object of the present invention to provide a capillaryrheometer which utilizes a pressure sensor for sensing pressure exertedby the melted polymer.

SUMMARY OF THE INVENTION

To accomplish the foregoing objects, features and advantages of thepresent invention, there is provided a capillary rheometer apparatuswhich includes a housing, a plunger, the housing having a reservoir forreceiving the plunger and a polymer melt, and means for blocking flow ofthe melt out of the reservoir. The rheometer further includes a drivingmechanism for driving the plunger longitudinally within the reservoir tomove one end of the plunger in contact with the melt. A diaphragm, whichis coupled to the one end of the plunger, deflects in response to meltpressure in the reservoir. The rheometer further includes a mechanism,responsive to the diaphragm deflection, for determining pressure of themelt.

In one embodiment of the present invention, the melt pressuredetermining mechanism includes an optical sensing mechanism. In anotherembodiment of the present invention, the melt pressure detectingmechanism includes a coupler at the one end of the plunger and aliquid-filled capillary passage extending within the plunger.

The capillary rheometer further includes a mechanism for determining thetemperature of the melt and a mechanism for indicating longitudinalmovement of the plunger.

BRIEF DESCRIPTION OF THE DRAWING

Numerous other objects, features and advantages of the invention shouldnow become apparent upon a reading of the following detail descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational partially broken view of a prior art embodimentof a force based capillary rheometer including a force sensor;

FIG. 2 is an enlarged cross-sectional view of the prior art force basedcapillary rheometer, illustrating in particular the force based plungerand the entrance to the capillary;

FIG. 3 is a cross-sectional view of the capillary rheometer of thepresent invention, illustrating use of the plunger pressure transducerassembly;

FIG. 3A is an exploded fragmentary view of the sensing diaphragm;

FIG. 3B is an exploded fragmentary view of the tip diaphragm;

FIG. 4 is a cross-sectional view of an alternate embodiment of thepresent invention illustrating the use of an additional pressure styletransducer;

FIG. 5 is a cross-sectional view of an alternate embodiment of thepresent invention illustrating a liquid metal filled, rigid stem,capillary rheometer plunger transducer;

FIG. 6 is a cross-sectional view of an alternate embodiment of thepresent invention illustrating a push rod, rigid stem, capillaryrheometer plunger transducer;

FIG. 6A is an enlarged, fragmentary, cross-sectional view of the pushrod, rigid stem, plunger transducer of the capillary rheometer of FIG.6;

FIG. 6B is an enlarged, fragmentary, cross-sectional view of the pushrod, rigid stem, plunger transducer of the capillary rheometer of FIG.6;

FIG. 7 is a cross-sectional view of an alternate embodiment of thepresent invention illustrating a non-bonded piezo resistive type, rigidstem, capillary rheometer plunger transducer;

FIG. 7A is an enlarged, fragmentary, cross-sectional view of thenon-bonded piezo resistive plunger transducer of the capillary rheometerof FIG. 7;

FIG. 8 is a cross-sectional view of an alternate embodiment of thepresent invention illustrating use of a heater block holder for thecapillary rheometer plunger transducer;

FIG. 9 is a cross-sectional view of an alternate embodiment of thepresent invention illustrating a PVT capillary rheometer apparatus whichutilizes the plunger pressure transducer assembly;

FIG. 10 is a cross-sectional view of an alternate embodiment of thepresent invention which utilizes a plunger pressure transducer assemblyhaving an optical arrangement for sensing the pressure of the meltedpolymer;

FIG. 11 is a cross-sectional, partly schematic view of the opticalsensing arrangement of FIG. 10; and

FIGS. 12 and 13 are curves illustrating the PVT behavior for certaintypical polymers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention there is provided a capillaryrheometer which utilizes a plunger pressure transducer assembly. Thisplunger pressure transducer assembly has a plunger with one end forforcing a melted polymer through a capillary and a diaphragm at the sameend of the plunger for sensing the pressure in the polymer. Itadditionally has a capillary passage with a liquid metal fill fluidtherein as well as another sensing diaphragm, located at the oppositeend of the pressure transducer assembly from the plunger with a strainsensitive element bonded to the surface opposite the liquid metal fill.As the plunger is lowered and pressed onto the top of the meltedpolymer, generating a pressure internal to the melted polymer, thediaphragm at the tip of the plunger, nearest the melted polymer,deflects with the melted polymer pressure, and transmits this deflectionto the liquid metal fill fluid in the plunger pressure transducerassembly. The other sensing diaphragm at the opposite end of the plungerpressure transducer assembly deflects with the pressure within theliquid metal fill fluid varying the resistance of the bonded strainsensitive element, yielding an accurate pressure measurement immune toany of the barrel friction related errors common to force based plungermeasurement techniques.

Reference is now made to the drawings and, in particular, to FIGS. 1 and2 in which a prior art embodiment of the force based capillary rheometeris illustrated. A preferred embodiment of the present invention,illustrating the capillary rheometer with the pressure plungertransducer is shown FIG. 3. Alternate embodiments of the presentinvention, utilizing the plunger pressure transducer, are illustrated inFIGS. 4-11.

FIGS. 12 and 13 illustrate the pressure-volume-temperaturecharacteristics of two different polymers.

Referring now in particular to FIGS. 1 and 2, which illustrate astandard force based type capillary rheometer, the force sensor 4 can beseen for measuring force of the plunger 5. Due to the aforementionederrors associated with this method, the present invention utilizes apressure transducer assembly 25, replacing the force based measurementplunger, as illustrated in FIG. 3.

Force based plunger type capillary rheometers, as illustrated in FIGS. 1and 2, use a piston or plunger 5 to force melted polymers, that havebeen heated in-situ, through a capillary die 12. The force, or meltpressure (calculated using the force measured by the force sensor 4divided by the effective area of the plunger 5 required to maintainsteady flow through the capillary die 12 at a given plunger velocity) ismeasured, and is indicative of the polymers' apparent shear viscosity.

The force based plunger-barrel capillary rheometer utilizes a forcesensor 4 to measure the load applied to the plunger 5 in order tomaintain a given plunger 5 velocity through the stationary barrel 6. Theapparent shear viscosity of the melted polymer 9 can be determined usingthe relationships for flow of polymer melts through cylindricalgeometries (i.e. pipe pressure flow). The apparent shear viscosity ofthe polymer melt at a given melt temperature and pressure, at the wallof the capillary 12, is determined by the ratio of wall shear stressdivided by apparent wall shear rate, for the capillary 12 of definedgeometry. The pressure gradient along the length of the capillary 12 isindicative of the shear stress. The discharge pressure of the capillary12 is assumed to be zero, so the pressure gradient is the capillary 12entrance pressure divided by the capillary 12 length. The apparent shearrate at the wall of the capillary 12 is calculated from the meltedpolymer 9 flow rate through the capillary 12, which is determined bymonitoring the position of the piston by means of a displacement sensor2 in the barrel with respect to time assuming melted polymer 9incompressibility and mass balance.

Also illustrated in FIGS. 1 and 2 is the load screw 1 or the like whichcan be driven by electromechanical or servohydraulic/electromechanical,servohydraulic-pneumatic means, or using weights and the force ofgravity. The problem with using weights, however, is that perfectalignment is necessary but difficult to achieve in practice, which inturn causes excessive friction error. In addition, the support columns 3are shown for supporting the plunger 5 and barrel 6. In addition, asupport bracket 7 is shown supporting the barrel 6 between the supportcolumns 3. Also shown are the plunger seals 8 for containing the meltedpolymer 9 within the barrel 6. In addition, the heater 10 is shown forheating of the barrel 6, as well as temperature sensors 11 fortemperature detection thereof.

The forced based plunger-barrel capillary rheometer may also be used todetermine the compressibility of the melted polymer. Instead of usingthe piston or plunger 5 to force the melted polymer through a capillarydie 12, a plug is used to maintain the melted polymer within the barrel6. The compressibility of the polymer melt can be determined from therelationship between the pressure sensed and the plunger position.

The aforementioned errors associated with these force based capillaryrheometers, however, render them inaccurate.

The present invention provides a capillary rheometer in which theaforementioned errors and corrective techniques are avoided. FIG. 3illustrates a preferred embodiment of the capillary rheometer of thepresent invention in which a pressure transducer assembly plungerreplaces the old force based measurement plunger. The capillaryrheometer, as shown in FIG. 3, consists of a barrel 6 heated by anelectrical power-controlled heater 10 with an appropriate capillary 12retained at the bottom. The plunger 5 (as shown in FIG. 2) has beenreplaced by a plunger pressure transducer assembly 25. The plungerpressure transducer assembly 25 is moved downward by the motor, a deadweight, or a pneumatic, mechanical, or hydraulically driven drive head,in a controlled rate of descent or at a constant stress. It is to beappreciated that pneumatic rheometers typically employ a constantpressure rather than a constant speed as in the motorized type. Thediaphragm 22 of the plunger pressure transducer assembly 25 presses ontothe top of the melted polymer 9 generating a pressure internal to themelted polymer 9 and the liquid metal fill fluid 21, as will bedescribed below. The plunger seal 8 prevents the melted polymer 9 fromescaping around and past the plunger pressure transducer assembly 25 andout the top of the barrel 6 and the associated seal friction is notconsidered in the pressure measurement. Melted polymer 9 begins to flowthrough the capillary 12 in a calculable manner. The tip diaphragm 22transmits the melted polymer pressure, in this configuration, to a fillliquid metal fluid 21 within the metal capillary 14 in the plungerpressure transducer assembly 25. The sensing diaphragm 19 deflects inresponse to the transmitted pressure of the liquid metal fluid 21,straining the four strain sensitive resistive elements within straingage 20. The four strain sensitive resistive gage elements are arrangedin a Wheatstone bridge configuration, with two increasing and twodecreasing resistive elements. The strain induced resistive changes arethen transformed into a voltage change. The voltage change is directlyproportional to the pressure change in the Capillary Rheometer barrel 6and inversely proportional to the voltage supplied to the Wheatstonebridge. Further details of the sensing diaphragm are illustrated in theexploded fragmentary of FIG. 3A. Similarly, further details of the tipdiaphragm 22 are illustrated in the exploded fragmentary view of FIG.3B.

In accordance with this preferred embodiment of the present invention,as illustrated in FIG. 3, further details of the plunger transducerassembly 25 will be described below. The metal capillary 14 can be seenwithin the metal armor flex hose 13 for flexible movement. The metalcapillary 14 encloses the liquid metal fill fluid 21. Tube 14 is weldedat 23 to plunger 5 and metal case 17 at its ends. Tube 14 is then filledand capped off with diaphragms 22 and welds 23. The measurementdiaphragm assembly 15 acts to measure the pressure of the liquid metalfill fluid 21 within the metal capillary 14. The measurement diaphragmassembly 15 includes the temperature compensation printed circuit boardassembly 16. Strand gage 20 is attached to circuit board 16 via flexiblecircuit board 24. This measurement diaphragm assembly 15 is enclosed inmetal case 17. An electrical connector 18 is provided on the peripheryof the metal case 17.

In an alternate embodiment of the present invention, in order tomaintain thermal stability and minimize temperature induced errors inthe plunger transducer assembly 25 during operation with the capillaryrheometer, a heater block holder is utilized, as illustrated in FIG. 8.The capillary rheometer plunger transducer rests within a holder 32,which is heated by an electrical heater 10 to the temperature of thepolymer under test, measured by the temperature sensor 11 and controlledby a conventional temperature controller (not shown). The base 34supports the outer cylindrical shell 30, which acts as a heat shield forthe holder 32 and the heater 10. The upper 31 and lower 33 platessupport and maintain centrality, respectively, of the holder 32 andprovide a plenum for air circulation from the holes provided in theouter cylindrical shell 30 through to the lower 33 and upper 31 plates.

The plunger pressure transducer assembly 25 is placed in the holder 32during purging, cleaning, reloading and packing of the polymer undertest in the capillary rheometer. The plunger pressure transducerassembly 25 is removed from the holder 32, inserted into the capillaryrheometer barrel 6 and allowed to thermally stabilize for a short periodof time prior to testing. With the plunger pressure transducer assembly25, pressure measurements are made as opposed to force based plungerswith which force measurements are made. Thus, the implementation of aplunger transducer assembly 25 into a forced based type capillaryrheometer, eliminates errors related to the seal frictional forcecomponent. In addition, the implementation of a plunger transducerassembly 25 into a forced based capillary rheometer eliminates theclearance area uncertainties from the pressure measurement calculationsrequired to establish a polymeric material's shear viscosity. Bettersealing can be achieved and therefore lower shear rate uncertaintyachieved, since the improved sealed quality can be used with noinfluence on the measured pressure value.

The viscosity of the polymer in the barrel 6 of the capillary rheometercan be determined using the plunger transducer assembly 25 (i.e., theviscosity of the polymer at shear rates lower than those encountered inthe primary capillary) if the difference between the plunger 5 andbarrel 6 discharge pressure can be measured.

The addition of another melt pressure style transducer 26, as shown inFIG. 4 with a rheometer which utilizes a pressure transducer before thecapillary die would allow the measurement of the pressure difference. Itshould be appreciated, however, that the use of the combinationplunger/pressure transducer in conjunction with a rheometer whichutilizes a pressure transducer before the capillary dye does not offerthe advantages that it does when implemented in a standard force basedcapillary rheometer, since the barrel pressure drop or plunger frictionerrors are not encountered with this rheometer. The use of such adevice, however, with the rheometer which utilizes a pressure transducerbefore the capillary die would allow one to evaluate viscosity at lowbarrel and high capillary shear rates at each plunger speed since thebarrel itself can be considered a large diameter capillary.

Barrel reservoir pressure drop (or head effect) is one of the factorsthat contributes to the force reading for piston rheometers whichutilize compressive load sensors at the upper end of the piston. Thebarrel pressure drop error is described as being significant. Theexistence of this error has in fact influenced certain rheologicalmeasurement practices.

Instruments such as an extrusion plastometer require that measurementsmust be made within certain piston height limits.

Development of piston rheometers which utilize pressure transducers atthe entrance to the capillary die eliminate the pressure drop errorbecause measurements are downstream from the barrel.

The barrel pressure drop is equivalent to: ##EQU1## where: Q_(B) =volumeflow rate through the barrel

μ_(B) =viscosity of the material in the barrel

R_(B) =radius of the barrel (inner)

L_(B) =effective length of the barrel (the distance between the pistontip and capillary entrance).

while the capillary pressure drop is equivalent to: ##EQU2## where:Q_(C) =volume flow rate through the capillary

μ_(C) =viscosity of the material in the capillary

L_(C) =length of the capillary

R_(C) =radius of the capillary

For a Newtonian, uncompressible fluid, the ratio of the barrel pressuredrop to the capillary pressure drop (which is an indicator of themagnitude of the error) is equivalent to: ##EQU3##

The error decreases as the test progresses because the effective lengthof the barrel decreases continuously throughout the test.

Most plastic materials are pseudoplastic in nature, having viscositiesthat decrease with increasing shear rate. For non-Newtonian materials,such as plastic melts, this ratio ##EQU4## where μ_(B) >μ_(C) and forhighly pseudoplastic polymers, μ_(B) >>μ_(C), since the shear rates inthe larger diameter barrel are must lower than those in the typicallysmaller diameter capillary at the same volume flow rate. The barrelpressure drop error is, therefore, more significant for pseudoplasticmaterials (for a given rheometer and capillary geometry) than forNewtonian materials.

The alternate embodiment capillary rheometer, as shown in FIG. 4,utilizes two pressure transducers, one being integral to the plunger,the other being placed at the capillary die entry. The difference in thetwo pressure readings is the barrel pressure drop, ΔP_(B). Using thissystem, the apparent shear viscosity of the material in the barrel, andthe viscosity of the material in the capillary (subject to the usualcapillary end error correction) can be calculated simultaneously.

Barrel ##EQU5## where μ_(a),B =apparent melt shear viscosity in thebarrel at ##EQU6## apparent shear rate. Q=volume flow rate, R_(B)=barrel radius ##EQU7## Capillary ##EQU8## L_(C) =capillary lengthΔP_(C) =capillary pressure drop

The apparent melt viscosity of the polymer is determined at two shearrates for each plunger speed (flow rate) with this system. The melt flowcharacteristics of the polymer are evaluated over a wider range of shearrates than can be evaluated utilizing a conventional force basedcapillary rheometer.

Alternate embodiments of capillary rheometer utilizing plungertransducer assemblies are illustrated in FIGS. 5, 6 and 7 and 9.

FIG. 5 shows the implementation of a liquid metal filled, rigid stem,capillary rheometer plunger transducer. As can be seen in FIG. 5, themetal case 17, enclosing the measurement diaphragm assembly 15, isattached directly to the plunger 5, rather than from the interim metalarmor flex hose 13. This alternate arrangement is thus referred to as a"rigid stem" system.

FIG. 6 shows the implementation of a push rod, rigid stem, capillaryrheometer plunger transducer. As in FIG. 5, this system is a rigid stemsystem. The alternate embodiment of FIG. 6 also includes a push rod 27within the plunger transducer assembly 25. The push rod 27 is indicatedpredominantly in FIG. 6.

FIG. 7 shows the implementation of a non-bonded piezo resistive type,rigid stem, capillary rheometer plunger transducer. This alternateembodiment, like the embodiments in FIGS. 5 and 6, is a rigid stemsystem. The alternate embodiment in FIG. 7, however, includes ameasurement diaphragm 29 consisting of either a highly elasticnon-metallic monocrystalline structure or a polycrystalline structure.Also shown in FIG. 7 are the high-temperature electrical connections 28for communication with the strain gage 20. Further details of themeasurement diaphragm 29 and high-temperature electrical connections 28,which communicate with the strand gage 20, are illustrated in theenlarged, fragmentary, cross-section view of FIG. 7A.

FIG. 9 illustrates a capillary rheometer apparatus utilizing the plungerpressure transducer assembly of the present invention. A capillaryrheometer can be used to determine the pressure-volume-temperature (PVT)behavior of a polymeric material. The material is placed in atemperature controlled chamber, allowed to reach thermal equilibrium,and the volume of the polymer in chamber is measured as pressure isapplied to the polymer through an instrumented piston. The testprocedures are repeated at various barrel temperatures and pistonpressures. The specific volume of the polymer, at various pressures, isplotted against the polymer temperature to describe the PVT behavior ofthe polymer, as described below. As shown, in FIG. 9, an electricallyheated stationary barrel 50 includes a polymer reservoir 62. Plug 54 isfixed to the lower portion 55 of barrel 50, which plugs the lowerportion of reservoir 62. A predetermined mass of polymer granules,pellets, powder or liquid is loaded into the reservoir 62 within barrel50. Barrel 50 is heated by barrel temperature controller 68 having probe70, causing the polymer 60 to melt. A plunger pressure transducerassembly 64 includes an instrumented piston 66 and an integral pressuresensor 72. A high pressure seal 65 exists between the instrumentedpiston 66 and inner walls 67 of reservoir 62. Mechanism 74 is used toapply force to the top of the plunger pressure transducer assembly 64.Mechanism 74 may include a dead weight or electrically driven,hydraulically driven, or pneumatically driven load screw, or the like.The plunger pressure transducer assembly 64 applies, through theinstrumented piston 66, pressure to the polymer melt 60 within reservoir62. Plunger pressure transducer assembly position sensing device 69senses the change in axial position of the piston 66. The positionsensing device 69 may include an optical encoder or LVDT. Pressuresensor 72 senses the pressure applied to the polymer melt 60. Melttemperature sensor 56, including probe 58, senses the temperature of thepolymer melt 60. Melt temperature sensor 56 may include an extendedthermal couple RTD or an infrared transducer. In addition, as shown, theapparatus includes insulation 52 on the outer sides of barrel 50 toprevent the outer sides of the barrel from reaching extremely hightemperatures. Once a desired pressure level is reached, the pressure ismaintained until steady state conditions are reached with respect totemperature and position. The procedure is repeated at various desiredtemperatures and pressure levels and temperature, position and pressureresults are recorded.

Based on the test results, the relationship between the polymer's melttemperature, pressure, and specific volume is established. Specificvolume (volume/mass) at any given pressure and temperature is determinedby comparing the change in volume (volume swept by the piston) to thevolume at atmospheric pressure. The relationship between the polymer'svolume, temperature and pressure (PVT behavior) determines the behaviorof the polymer material and influences the quality of products obtainedfrom a manufacturing process utilizing such polymer material, such asinjection molding. A polymer's PVT behavior is typically characterizedfor either process design or material quality control purposes. Thus, acapillary rheometer apparatus which can accurately yield accurate PVTinformation, is desired.

The PVT relationship for a polymeric material is generally given by:

    (p+π)(v-β)=RT

where:

p=melt pressure

π=a material constant

v=specific volume of the melt (volume/mass)

β=a material constant

R=universal gas constant

T=melt temperature

The PVT behavior for a typical amorphous polymer such as polystyrene isillustrated in FIG. 12. FIG. 12 shows that the specific volume of anamorphous polymer increases with increasing temperature, and decreaseswith increasing pressure. The curve in FIG. 12 also shows that the rateof change of specific volume with temperature and pressure increasesabove the glass transition temperature (T_(g)) of the polymer.

The PVT behavior of a semi-crystalline polymer such as polypropylene isshown in FIG. 13. FIG. 13 also shows that the specific volume of such apolymer increases with increasing temperature, and decreases withincreasing pressure. The PVT behavior of a semi-crystalline polymerdiffers from that of an amorphous polymer in that there is a majordiscontinuity in each specific volume vs. temperature curve (isobar) atthe polymer's melting temperature (T_(m)).

The capillary rheometer apparatus of FIG. 9 can also be used in theunsteady state mode to evaluate the specific heat of a polymer bylooking at the temperature increase associated with compression assumingadiabatic conditions (adiabatic heating or cooling during instantaneouspressure changes). Fast response melt temperature probes and low thermalconductivity barrels are desirable for such an evaluation to minimizeheat loss. The adiabatic temperature rise is given by:

    ΔT=ΔP/ρC.sub.p

where:

Δt=instantaneous melt temperature change

ΔP=pressure change

ρ=density

C_(p) =specific heat at constant pressure

The specific heat is calculated based upon the measured variables, ρ,ΔT, and ΔP.

Use of the plunger pressure transducer assembly in the capillaryrheometer apparatus allows for direct measurement of both pressure andtemperature along the center of the molten polymer within the reservoir.The direct pressure measurement is advantageous over indirectmeasurements which involve friction and leakage. The quality of thepiston seal is particularly important in PVT applications sincepressures are high (potentially up to 30,000 psi). Tight piston sealswould result in high frictional forces. However, a stationary pressuresensor placed at the bottom of the barrel would work just as well,except that temperature could not properly be measured in that somelocation. It will be appreciated by those skilled in the art thatpressures could be measured at right angles to the melt flow. This couldbe done (1) at the reservoir wall (which would change the volume of thereservoir since the sensor has a flat face) of (2) through aninterconnecting hole (which would change the volume of the chamber ofthe reservoir and introduce hole pressure errors). Thus, such rightangle measurement is not preferred.

FIGS. 10 and 11 show an alternate embodiment of the plunger pressuretransducer assembly 73 which includes an optical pressure sensingarrangement. As shown, the optical sensing arrangement includes an inputoptical fiber 75, diaphragm 78 and an output optical fiber 76. Thediaphragm 78 deflects in response to pressure by the polymer melt 60within the reservoir 62. The diaphragm includes a moveable reflector 80attached thereto and a fixed reflector 82. When the diaphragm 78deflects in response to pressure, the moveable reflector 80 movestherewith. One skilled in the art will appreciate that the opticalpressure sensing arrangement plunger assembly 73, while shown in FIGS.10 and 11 for use in the apparatus of FIG. 9, can be used also with theapparatus shown in FIG. 3.

The optical sensing mechanism operates as follows. Input light from aninput source 84 is coupled into an input optical fiber 75, reflects offof movable reflector 80, then reflects off of fixed reflector 82 and iscoupled into an output optical fiber 76. The light coupled into outputoptical fiber 76 is detected by photodetector 86. The amount of lightcoupled into an output optical fiber varies with the position of themovable reflector 80. Because the movable reflector moves in response topressure in the liquid melt 60, the optical sensing mechanismdetermines, from the amount of light detected at the photodetector 86,the amount of pressure of the liquid melt 60. Photodetector 88 receivesa small predetermined reflected portion of the input light generated byinput light source 84 to monitor the amount of input light generated. Afeedback arrangement (not shown) is connected from photodetector 88 toinput light source 84 to control input light source 84. In that manner,a maximum constant amount of input light is coupled into an inputoptical fiber 74 thereby optimizing the accuracy and stability of theoptical pressure sensing arrangement. Input light source 84 can be anLED. The optical sensing mechanism is described in U.S. patentapplication Ser. No. 07/907,331 entitled "OPTICAL PRESSURE TRANSDUCER"filed Jul. 1, 1992, which is herein incorporated by reference.

It is to be appreciated, that the use of the plunger transducer assemblyand instrumented piston, as opposed to the prior art force basedplunger, eliminated friction errors and reduces leakage errors withoutinfluencing the measured pressure. Additionally, the alternateembodiment optical pressure measuring apparatus produces a highlyaccurate pressure measurement.

It is to be appreciated that the preferred embodiment of the presentinvention utilized a plunger pressure transducer assembly in a forcebased type capillary rheometer which allows for the determination ofmelted polymer material properties without certain errors associatedwith the force based type capillary rheometer, but the plunger pressuretransducer assembly is not limited to use in a force based typecapillary rheometer.

Having now described a limited number of embodiments of the invention,it should not be apparent to those skilled in the art that numerousembodiments and modifications thereof are contemplated as falling withinthe scope of the present invention as defined by the appended claims.

What is claimed is:
 1. A capillary rheometer apparatus forcharacterizing a polymer melt's pressure-volume-temperaturerelationship, comprising:a housing; a plunger; the housing having areservoir for receiving the plunger and the polymer melt; a plug forblocking flow of the melt out of the reservoir; a driving mechanism fordriving the plunger longitudinally within the reservoir; the plungerhaving a liquid-filled capillary passage extending therein from one endof the plunger; a coupler at the one end of the plunger defining withinthe plunger a chamber in communication with the capillary passage andfor sensing melt pressure in the reservoir and transmitting the pressureto the liquid-fill; and a pressure sensing mechanism, at a second end ofand coupled to the capillary passage, responsive to pressure exerted bythe liquid fill, for providing an indication of sensed pressure.
 2. Acapillary rheometer as claimed in claim 1 further including atemperature controller, coupled to the housing, for controlling thetemperature of the housing.
 3. A capillary rheometer as claimed in claim1 further including a position detecting sensor, coupled to the plunger,for detecting longitudinal movement of the plunger.
 4. A capillaryrheometer as claimed in claim 1 further including insulation materialsurrounding said housing for insulating the housing.
 5. A capillaryrheometer as claimed in claim 1, wherein the plug includes a temperaturesensor for sensing a temperature of the polymer melt.
 6. A capillaryrheometer as claimed in claim 5, wherein the temperature sensor includesa probe inserted into an aperture in the reservoir.
 7. A capillaryrheometer apparatus for characterizing a polymer melt'spressure-volume-temperature relationship, comprising:a housing; aplunger having a piston at one end thereof; the housing having areservoir for receiving the piston and the polymer melt; means forblocking flow of the melt out of the reservoir; a driving mechanism fordriving the plunger and piston longitudinally within the reservoir;means, coupled to the plunger, for determining pressure of the melt;means, coupled to the reservoir, for determining the temperature of themelt; and means, coupled to the plunger, for sensing a longitudinalposition of the plunger.
 8. A capillary rheometer as claimed in claim 7wherein the means coupled to the plunger includes an instrumented pistonattached to the one end of the plunger.
 9. A capillary rheometer asclaimed in claim 8 wherein the instrumented piston includes a seal forsealing space between the piston and reservoir.
 10. A capillaryrheometer as claimed in claim 7 further including means, coupled to thehousing, for controlling the temperature of the housing.
 11. A capillaryrheometer as claimed in claim 10 wherein the means for blocking flowincludes a plug disposed within an opening in the housing at one side ofthe reservoir.
 12. A capillary rheometer as claimed in claim 11 whereinthe piston includes a seal thereon for sealing space between the pistonand reservoir.
 13. A capillary rheometer as claimed in claim 7 whereinthe means for determining pressure of the melt includes an opticalpressure sensing mechanism.
 14. A capillary rheometer as claimed inclaim 7 wherein the means for determining pressure of the melt includesthe piston attached to the plunger and a liquid-filled capillary passageextending within the plunger.
 15. A capillary rheometer apparatus forcharacterizing a polymer melt's pressure-volume-temperaturerelationship, comprising:a housing; a plunger; the housing having areservoir for receiving the plunger and the polymer melt; a plug forblocking flow of the melt out of the reservoir; a driving mechanism fordriving the plunger longitudinally within the reservoir to move one endof the plunger in contact with the melt; a diaphragm, coupled to one endof said plunger, for deflecting in response to melt pressure in thereservoir; and means, responsive to diaphragm deflection, fordetermining pressure of the melt.
 16. A capillary rheometer as claimedin claim 15 wherein the means for determining melt pressure includes anoptical sensing mechanism.
 17. A capillary rheometer as claimed in claim16 further including:means, coupled to the reservoir, for determiningthe temperature of the melt; and means, coupled to the plunger, forindicating longitudinal movement of the plunger.
 18. A capillaryrheometer as claimed in claim 16 wherein the optical sensing mechanismincludes an input optical fiber, an output optical fiber, a fixedreflector, a movable reflector coupled to the diaphragm, a light source,and a light level detector.
 19. A capillary rheometer as claimed inclaim 15 wherein the means for determining melt pressure includes thediaphragm at the one end of the plunger and a liquid-filled capillarypassage extending within the plunger.
 20. A capillary rheometer asclaimed in claim 19 further including:means, coupled to the reservoir,for determining the temperature of the melt; and means, coupled to theplunger, for indicating longitudinal movement of the plunger.
 21. Amethod for characterizing a polymer melt's pressure-volume-temperaturerelationship, comprising:introducing the polymer material into areservoir contained within a housing; heating the housing to melt thepolymer; applying pressure to the polymer with a plunging apparatusintroduced into an aperture of the reservoir; measuring a pressure ofthe polymer melt with a pressure sensing apparatus coupled to theplunger; and measuring a temperature of the polymer melt with a probeintroduced into the reservoir.
 22. The method of characterizing apolymer melt at claimed in claim 21, further comprising the stepsof:repeating the steps of applying pressure, measuring the pressure, andmeasuring the temperature for a number of pressures applied to thepolymer melt.